The average cost of some books is $80.
When another 8 books are added,
the total cost increases by $330.
The average cost becomes $70.
How many books are there in the end?
|
Case 1 |
Case 2 |
Number |
1 u |
1 u + 8 |
Value |
80 |
70 |
Total value |
80 u |
70 u + 560 |
Number of books at first = 1 u
Number of books in the end = 1 u + 8
Total cost of books at first
= 1 u x 80
= 80 u
Total cost of the books in the end
= 70 x (1 u + 8)
= 70 u + 560
80 u + 330 = 70 u + 560
80 u - 70 u = 560 - 330
10 u = 230
1 u = 230 ÷ 10 = 23
Number of books in the end
= 1 u + 8
= 23 + 8
= 31
Answer(s): 31